1. Field of the Invention
This invention relates to morphologically anisotropic ultrafine particles of aluminum nitride having the form of columns, plates, or whiskers, a method for the production thereof, and a sintered article obtained by sintering a powder containing the ultrafine particles of aluminum nitride mentioned above. More particularly, the present invention relates to composite ultrafine particles comprising morphologically anisotropic ultrafine particles of aluminum nitride and ultrafine particles of rare earth nitride, which are usable as a raw material for the production of a sintered article, a reinforcing material for various metal-based composite materials, a blast powder, and the like.
2. Description of the Prior Art
The sintered articles of aluminum nitride enjoy resistance to heat, high thermal conductivity, high electrical insulating quality, and light transmission properties and, by virtue of these features, find extensive utility in various applications as to heat-releasing substrates for semiconductors, printed circuit boards, packaging materials for LSI (large scale integrated circuit), light-pervious hear-resistant materials, and the like. Though these sintered articles of aluminum nitride are commonly produced by sintering aluminum nitride powder, the properties of the produced sintered articles are affected largely by the purity and particle diameter of a particular powder used as the raw material and further by the purity, particle diameter, and manner or mixture of a sintering auxiliary to be added to the raw material. For tile purpose or obtaining a sintered article of high performance, it is necessary that the aluminum nitride powder to be used should possess high purity and fineness and the sintering auxiliary powder to be additionally used should possess high purity and fineness and should be mixed uniformly with the aluminum nitride powder.
As concrete examples of the methods heretofore used for the production of aluminum nitride powder, the following methods may be cited:
(1) a method which comprises directly nitriding metallic aluminum and then finely pulverizing the resultant aluminum nitride, PA1 (3) a method which comprises attaining vapor-phase synthesis of aluminum nitride powder by utilizing the reaction of an aluminum halide with ammonia, and PA1 (4) a method which comprises melting metallic aluminum with arc or plasma in an atmosphere of a mixed gas of nitrogen and ammonia thereby preparing a mixture of ultrafine particles of metallic aluminum and aluminum nitride and then heating the ultrafine particles at a high temperature in an atmosphere of nitrogen thereby nitriding ultrafine particles of metallic aluminum (see Japanese Patent Applications, KOKAI (Early Publication) No. 62-283,805 and No. 62-282,635).
(2) a method which comprises reducing aluminum oxide powder with carbon of high purity and meanwhile nitriding the resultant reduction product by reaction with nitrogen,
The methods (1) and (2) mentioned above, however, do not easily produce aluminum nitride powder having a particle size of not more than 1 .mu.m and the method (3) mentioned above is at a disadvantage in obtaining aluminum nitride powder of high purity only with difficulty. The method (4) mentioned above has the problem of being incapable of obtaining a sintered article of aluminum nitride of high performance because the produced particles are not wholly nitrided and further because the ultrafine particles of metallic aluminum which have escaped being nitrided are extremely active and induce ignition or oxidation on exposure to even a small amount of oxygen. Further, the aluminum nitride particles obtained by the four methods mentioned above have relatively isotropic forms such as hexagons or spheres. When these particles are manufactured into a the sintered article or they are used as a reinforcing material in the production of a composite material having a metallic matrix, the sintered article or the composite material has room for further improvement in strength, particularly hot strength.